1. Field of the Invention
The present invention relates to an ultrasonic sensor for use in obstruction detectors, automatic doors and the like. The present invention also relates to an obstruction detector which has the ultrasonic sensor.
2. Description of Related Art
One example of a conventional obstruction detector is a corner sonar that includes an ultrasonic sensor attached to a front bumper of a vehicle and a controller connected to the ultrasonic sensor.
Utilizing the resonance phenomenon exhibited by an ultrasonic resonator, the ultrasonic sensor emits ultrasonic waves. The resonance phenomenon may be generated based on a resonance voltage from the controller.
When ultrasonic waves reflected from an obstruction in the forward direction is received by the ultrasonic resonator, the ultrasonic sensor emits the reflected ultrasonic waves as the reception signal to the controller using a resonance circuit that includes the ultrasonic resonator and a coil of a transformer. After receiving the reception signal, the controller compares the reception signal with a predetermined threshold level and determines if there is an obstruction or not. It must be noted that the ultrasonic sensor generates the reception signal when it is not receiving the resonance voltage from the controller.
However, in such conventional obstruction detector, changes in the electromagnetic induction field in the vicinity of the ultrasonic sensor may affect the coil of the transformer. That is, the coil might generate a counter electromotive force due to fluctuations in the electromagnetic induction field. Thus, the coil might provide the reception signal that contains noise components (which correspond to the counter electromotive force) to the controller. In this way, because the reception signal contains noise components, the controller might erroneously determine that there is an obstruction even if there is actually none.